Tokyo has four real seasons: pack a warm coat for December to February (highs around 10–12°C), light clothes and an umbrella for humid summers (31°C in August), and layers for spring and autumn.
Tokyo is a walking city, and a 15,000-step day is normal. If you pack one thing with care, make it comfortable walking shoes, broken in at home, ideally easy to slip off for temples and restaurants that ask you to remove them.
After that, pack for the season: a packable down jacket for winter, a compact umbrella for the June rainy season and September typhoons. Japan still loves cash, and a little coin purse keeps the 500-yen coins under control. Packing cubes and a power bank for maps and translation apps round it out.
Comfortable walking shoes matter all year: Tokyo is a walking city. September and October are the wettest months, with typhoons possible. Japan uses Type A plugs at 100 V, so two-pin Canadian plugs fit, but three-prong plugs need an adapter.